If you really cared, you could probably improve it by a lot.
I'm just telling you what I would do. I would rop... no just kidding
First check your hormone levels, thyroid t3,t4, testosterone and estrogens. Then you can plan with your doctor if there are some hormonal imbalances.
Then start a strength routine which includes deadlifts, squats, benchpress, overheadpress and some sort of row. Then you train 3 times a week full body. On the off days you could do some active recovery. Learn the exercises first until they feel somewhat natural, then try to add a little weight every time.
Make sure that you sleep enough and somewhat consistently every night.
Count protein every day and try to get atleast 2g per kg bodyweight from animal sources, such as red meat for example. Fats and carbs you just eat such that you gain weight consistently, but not too fast. You can adjust the amount you eat based on the speed you gain weight.
If you do that I can almost guarantee you that you will gain a lot of muscle.